Heretofore, a waste heat recovery apparatus utilizing a flash cycle has been known. For example, FIG. 3 of the following Non-Patent Literature 1 discloses a waste heat recovery apparatus which comprises: a gas-liquid separator; an expander into which a gaseous working medium separated by the gas-liquid separator flows; an electricity generator connected to the expander; an expansion valve operable to expand a liquid working medium separated by the gas-liquid separator; a mixer for mixing the working medium discharged from the expander and the working medium after passing through the expansion valve; a condenser operable to condense the mixed working medium from the mixer; a pump operable to pressurize the working medium condensed by the condenser; and an evaporator operable to evaporate the working medium pressurized by the pump.
In the waste heat recovery apparatus described in the Non-Patent Literature 1, the liquid working medium separated by the gas-liquid separator passes through the expansion valve before merging with the working medium discharged from the expander. In this process, the liquid working medium is adiabatically expanded. That is, in this waste heat recovery apparatus, heat energy of the liquid working medium is discarded during the process in which the liquid working medium passes through the expansion valve. Moreover, in this waste heat recovery apparatus, the discarded heat energy of the liquid working medium is not effectively recovered. Thus, a rate of recovery as motive power in the expander, i.e., electricity generation efficiency in the electricity generator, is not sufficient.